Rosneft has bought a 30% stake in the Zohr gas field for $1.125 billion (£850m).
The Russian state-owned energy giant acquired the stake, which is the largest in the Egyptian sector of the Mediterranean Sea, from Italian company Eni S.p.A.
The agreement stipulates a strategic partnership between Rosneft and Eni, which will own a 60% stake, with BP owning the remaining 10% share.
The company hopes the move will strengthen its position in the European and Middle Eastern markets.
Rosneft CEO, Igor Sechin, said: “It opens up the opportunities for the company to reinforce its positions in the promising and strategic region, broadens potential of our trading division and enhances our mutually beneficial cooperation with Egypt.”